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This article provides information about the sizing and standardization of a Faraday cup (FC) used as a plasma diagnostic. This instrument is used to accurately map the ion beam profile produced by an electric propulsion (EP) device. A FC is a cylindrical probe that uses an electrode, termed collector, to measure the current. Several studies have shown the relevance of adding an extra electrode, called collimator, to define the collection area and to minimize interactions with the ambient plasma. Both the electrodes are encapsulated into an isolated metallic housing that prevents ambient plasma from disturbing the measurements. In this case study, a field-emission-electric propulsion (FEEP) thruster is used. The FEEP technology uses electrostatic fields to extract liquid metal (indium) ions from a sharp surface and accelerates them to high velocities, providing thrust. The FEEP model used in this study is the ENPULSION NANO thruster from the Austrian company Enpulsion. We present results focusing on the sizing of a FC in terms of cup length, aperture diameter, and collection solid angle as well as on the material exposure to the ion beam. For a far-field ion beam study of a FEEP indium based electric thruster, our study outcomes show that a FC optimum sizing is a 50 mm long collector cup and a 7 mm wide inlet aperture. Moreover, shielding the repeller/collimator from direct exposure to the ion beam seems to greatly minimize perturbation during ion current acquisition. Finally, to only measure the ion current, a negative potential should be applied to the collector and repeller, where the latter is more negative. This study contributes to the effort on diagnostic standardization for EP device characterization. The goal is to enable repetitive and reliable determination of thruster parameters and performances.
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Emissive and Langmuir probes are two widely used plasma diagnostic techniques that, when used properly, give access to a wide range of information on the plasma's ions and electrons. We show here that their use in small and medium power Hall thrusters produces large perturbations in the discharge characteristics. Potential measurements performed by both probes and non-invasive Laser Induced Fluorescence (LIF) spectroscopy highlight significant discrepancies in the discharge profile. This phenomenon is observed both in the 200 W and the 1.5 kW-class thrusters. In order to have a better understanding of these perturbations, ion velocity distribution functions are acquired by LIF spectroscopy at different positions in the smaller thruster, with and without the probes. Emissive probes are shown to produce the biggest perturbation, shifting the acceleration region upstream. The probe insertion is also shown to have significant effect on both the average discharge current, increasing it by as much as 30%, and its harmonic content in both amplitude and spectrum. These perturbations appear as the probe tip passes a threshold located between 0 and 5 mm downstream of the thruster exit plane.
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Time-resolved variations of the ion velocity distribution function (IVDF) are measured in the cylindrical Hall thruster using a novel heterodyne method based on the laser-induced fluorescence technique. This method consists in inducing modulations of the discharge plasma at frequencies that enable the coupling to the breathing mode. Using a harmonic decomposition of the IVDF, one can extract each harmonic component of the IVDF from which the time-resolved IVDF is reconstructed. In addition, simulations have been performed assuming a sloshing of the IVDF during the modulation that show agreement between the simulated and measured first order perturbation of the IVDF.
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This paper reports on the design, the building, the calibration, and the use of a compact E × B probe that acts as a velocity filter or a mass filter for ion species. A series of measurements has been performed in the discharge and in the beam of the PEGASES (Plasma Propulsion with Electronegative GASES) ion source. PEGASES is a unique inductively coupled radio-frequency source able to generate a beam of positive and negative ions when operated with an electronegative gas. In this study, experiments have been carried out with SF6. Calibrated E × B probe spectra indicate that the diagnostic tool can be used to determine the ion velocity and the plasma composition even when many molecular fragments are present. In addition, the probe is able to detect both positive and negative ions. Measurements show a large variety of positively charged ions coming from SF6. Conversely, the beam is solely composed of F(-) and SF6(-) negative ions in compliance with computer simulations.
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A novel approach to perform time-resolved laser-induced fluorescence (LIF) measurements in plasma discharges is presented. The LIF technique relies on a photon counting method associated with a sinusoidal potential modulation on a floating electrode located in the plasma to ensure time coherence. By tuning the modulation frequency, resonance can be reached with the discharge current in order to guarantee repeatable measurement conditions. Time-averaged characteristics of the discharge (such as Te, ne, Vp, and Vion) remain unaffected by the modulation. As an example, the association of the photon counting method with the modulation system is employed to determine the time evolution of several ion velocity groups inside an E × B discharge. Interesting features of the velocity oscillations are examined and pave the way for more focused studies.
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A compact high-speed reciprocating probe system has been developed in order to perform measurements of the plasma parameters by means of electrostatic probes in the discharge and the plume of a Hall thruster. The system is based on a piezoelectric linear drive that can achieve a speed of up to 350 mm/s over a travel range of 90 mm. Due to the high velocity of the linear drive the probe can be rapidly moved in and out the measurement region in order to minimize perturbation of the thruster discharge due to sputtering of probe material. To demonstrate the impact of the new system, a heated emissive probe, installed on the high-speed translation stage, was used to measure the plasma potential and the electron temperature in the near-field plume of a low power Hall thruster.
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The evolution of the ground-state hydrogen atom velocity distribution function throughout the stationary shock wave of a supersonic hydrogen plasma jet (3
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The transport of ground-state atomic hydrogen in the expansion of a thermal plasma generated from an Ar-H2 mixture is studied by means of laser-based diagnostic techniques. The flow of hydrogen atoms is investigated by two-photon excitation laser-induced fluorescence (LIF), whereas Ar atoms are probed by LIF as well as by UV Rayleigh scattering. The transport of Ar atoms can be fully understood in terms of a free jet flow; H atoms on the contrary exhibit an anomalous behavior. In the course of the plasma expansion, hydrogen atoms decouple from the argon fluid by a diffusion process as a direct consequence of recombination of H atoms at the vessel walls. In this contribution it is shown, on the basis of experimental results, how plasma-surface interactions can strongly influence the flow pattern of an atomic radical fluid.
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The time profile of the fluorescence light emission of atomic hydrogen in an expanding plasma beam after pulsed excitation with a nanosecond laser is studied, both experimentally and computationally. Ground state H atoms in an expanding Ar-H cascaded arc plasma are excited to the p=3 level using two-photon laser excitation at 205 nm. The resulting fluorescence is resolved in time with a fast photomultiplier tube to investigate the occurrence of quenching. A fluorescence decay time of (10+/-0.5) ns is measured under all circumstances, indicating that there is a complete l mixing of the p=3 sublevels. A time-resolved collisional radiative model is developed to model pulsed laser induced fluorescence for a large range of plasma parameters. The model calculations agree well with the experimental results over the entire range of conditions and indicate that two-photon LIF can strongly influence the local electron and ion densities, resulting in a "self-quenching" of the laser-induced H fluorescence.
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A two-photon laser-induced fluorescence study on the transport of ground-state atomic hydrogen in a supersonic plasma jet, generated from an Ar-H (2) mixture, reveals an unexpected shock pattern. Whereas both the axial-velocity profile and the temperature profile of hydrogen atoms along the jet centerline can be interpreted in terms of a supersonic expansion of an Ar-H gas mixture, the H-atom density profiles do not satisfy the well established Rankine-Hugoniot relation leading to a nonconservation of the forward flux. The experimental results show that H atoms escape from the supersonic expansion by a diffusion process due to strong density gradients between the core of the jet and its vicinity.